Uses and limits of thermal indices: the case of Sahel

International audience ; Our main goal here is to analyse extreme heat waves (HWs) in the Sahel (13°N-18°N; 16°W-30°E), using different thermal indices. In the ACASIS project funded bythe French “Agence Nationale de la Recherche”, HWs analyses are characterized forthe first time during the hottest season in the Sahel, using the Global Summary of theDay (GSOD) synoptic observations during April-May-June 1973-2013. Such extremehigh temperatures are usually defined by 3 criteria: 1/ Their low probability ofoccurrence: less than the 10% of the days. The use of absolute thresholds,associated with heat budget and physiological impacts, could be an alternative(Seneviratne et al., 2012). 2/ Their intensity, i.e. their amplitude in terms of standarddeviations from the local climatology (Goubanova, 2007), or otherwise excesses overabsolute intensity thresholds. 3/ Their severity: extreme events causing socioeconomicor human losses, the notion of impact is thus of major importance for theirdefinition (Beniston et al., 2007).According to prior IPCC assessments (TAR, AR4 and SREX), it is very likely thatincreased maximum temperatures and enhanced probabilities of hot days occurrenceswill occur at the global scale. There is also medium confidence that warm spells/heatweave frequencies, lengths or intensities will increase in many regions (Cubasch et al.,2013). Because of its latitude, Sahel is especially concerned, with diurnaltemperatures often exceeding 40°C in boreal spring and, to a lesser extent, autumn.AR4 concluded that global land-surface air temperature (LSAT) had increased over theinstrumental period of record, with the warming rate approximately double thatreported over the oceans since 1979. AR5 confirms previous estimates: Global LSATincreased: 0.1005°C ± 0.01925°C per decade over 1901-2012 period while the risereaches 0.262°C ± 0.05°C per decade over 1979-2012 time period, according to theobservations of CRUTEM4.1.1.0, GHCNv3.2.0, GISS and Berkeley dataset average(Hartman et al., 2013). Changes in the length of .